Through the ages, it has been known that a small region with the skin resistance lower than that of the surrounding regions exists on the skin surface, and the distribution thereof comparatively well corresponds with the distribution of acupuncture points as a stimulation point in acupuncture treatment. Such a region with the skin resistance lower than that of the surrounding regions (referred to as skin resistance attenuation point) is used for treatment in Ryodoraku (good electroconductive meridian) autonomic nerve modulation method, a low frequency treatment apparatus and the like. Apparatuses searching for the skin resistance attenuation points are available in the market, and are practically used in clinical diagnosis and treatment.
In the past, in the apparatus searching for (determining) an acupuncture point as the skin resistance attenuation point, a method in which the skin DC resistances are measured as electric characteristics of the skin in and around the acupuncture point, and the acupuncture point is determined based on the difference thereof has been often used. However, in determining the acupuncture point by measuring the DC current resistance as above, for example, there are following problems. That is, the measurement results depend on a contact state of the measurement electrodes, or the measurement result is not able to be clearly differentiated from spontaneous change of the skin voltage, resulting in poor reliability and poor reproducibility of the measurement results.
Therefore, in recent years, the following method has been employed as a method to improve the reliability and the reproducibility of the measurement results. In such a method, as electric characteristics of the skin in and around an acupuncture point, a distribution on the complex plane of the frequency response of the skin impedance (skin impedance locus) is measured based on a voltage generated by applying an AC current to the skin in and around the acupuncture point, and the acupuncture point is determined based on the difference of the skin impedance locus. For example, out of 4 parameters characterizing the shape of the skin impedance locus (after-mentioned Z0, Z∞, β, and τm), the central relaxation time τm in Cole-Cole circular arc's law is noted as a parameter that does not largely depend on the contact state of the measurement electrodes and clearly reflects the electric characteristics difference between the acupuncture point and the other regions. Based on the difference of the parameter τm, the acupuncture point is determined. Thereby, the problems included in the prior art can be avoided (for example, refer to the following Patent Document 1).
Patent Document 1: Japanese Patent Application Publication No. 2004-337349 (Abstract and claim 2)
In the prior art, in evaluating the electric characteristics difference between the acupuncture point and the other regions, the central relaxation time τm in Cole-Cole circular arc's law is calculated as a parameter characterizing the shape of the skin impedance locus. In the prior art, however, the following problems exist.
When the central relaxation time τm characterizing the shape of the skin impedance locus is calculated, the Cole-Cole circular arc's law should be satisfied in the frequency response of the skin impedance. However, the prior art has no means for confirming whether or not Cole-Cole circular arc's law is satisfied. Further, the central relaxation time τm should be inferred based on the measured skin impedance in a plurality of frequencies with the use of nonlinear least square method or the like. However, the procedure for inferring it is tangled, leading to a complicated hardware configuration, and thus such a method is not suitable for realizing a small size apparatus. Furthermore, to precisely infer the central relaxation time τm, as shown in FIG. 4, it is necessary that the measured skin impedance data is distributed not in an unbalanced manner but uniformly on the skin impedance locus.
However, as shown in FIG. 5, the distribution of the skin impedance data measured on actual skin often partially exists on the impedance locus in an unbalanced manner. In addition, the distribution manner often largely varies according to each measurement point. In this case, to precisely infer the central relaxation time τm characterizing the shape of the skin impedance locus, it is necessary to measure the skin impedance in the wide range of frequencies including extremely low frequencies. In result, the time resolution is lowered. In addition, as the measurement target frequency becomes lower, the measurement is subject to disturbance due to temperature drift, human physical movement and the like, and thus it is difficult to assure the reliability and the reproducibility of the measurement result.